The invention relates to a press platen or endless belt for embossing materials, in particular wooden materials or plastic materials, by means of surface structuring and a process for manufacturing the press platens or endless belts as per the invention.
Press platens or endless belts are required to press wooden plates with decor paper and/or overlay paper. The entire surface of the wooden plates is coated here with aminoplast resin films, also called thermoset resins. The processing takes place in single-daylight short-cycle presses, multi-daylight presses or double-belt presses, for example. The completed press platens or endless belts are used in the presses to provide the wooden materials that are to be manufactured with a desired surface structure. Registered embossing is used here as a preference, but not exclusively. This means that an assigned decor paper is used with an aligned press platen or an endless belt so that the structure of the decor paper can be raised in relief when embossed into the surface. The structures that are used in connection with this can have a natural origin, for instance an imitation of a wood-pore structure or stone surface. The possibility also exists to manufacture any desired structures in accordance with customer requests.
The decor and overlay paper is preferably made of thermoset resins, which transition into a liquid form because of the pressure and temperature in the press system and which form a secure bond with the wooden material with a rising level of cross-linkage and a simultaneous increase in the resin viscosity in the final state. Melamine resins, phenolic resins and melamine/urea resins, which have turned out to be especially suitable, are among the thermoset resins that are used. The melamine resin liquefies during the pressing process with pressure and heat, and further polycondensation takes place. The pressing time and temperature determine the degree of cross-linkage of the melamine resins and their surface quality. After the pressing period ends, the melamine resin has the desired degree of cross-linkage and has transitioned into a solid phase; a realistic surface design that reproduces the surface of the embossing tool is obtained via the simultaneous structured embossing of the melamine resin surface.
In the prior art, embossing tools in the form of press platens or endless belts are made of sheet steel; they get the required surface structure via appropriate processing. Different methods are known for this, for instance the creation of an etch resist by applying a screen-printing process so that subsequent etching of the press platen can be done. The plates that are used in connection with this have a very large format, so very precise processing and, in particular, further aligned processing is important if further work steps are required. There are always several work steps with especially deep structures have to be etched and several etching operations are required for this with a previously applied etch resist. All of the areas that are supposed to form the raised areas of the surface structure later on are covered by the mask in the etching processes, so the surface will only be etched in the areas that can be directly corroded by the etching fluid. The etched-out areas then form the profile valleys of the desired structure; the surface is cleaned and, in particular, the mask is removed after the end of the etching process.
Alternatively, the possibility exists to apply a photo layer that is subsequently exposed to light in order to subject the press platens or the endless belts to an etching process after the subsequent development of the photo layer, where a portion of the photo layer remains as an etch resist. It is very difficult and problematic to reproduce the masks that are created in this way, because the etch resist layers that are to be applied have to always be arranged in the same position when especially deep structures are to be etched.
Furthermore, instead of a screen-printing process, the creation of a mask via the application of wax or alternatively the use of a UV lacquer, which is directly printed onto the press platens and endless belts to be processed with the aid of a digital printing process, is known from the state of the art. After the etching, which may have to be repeated a number of times, especially deep structures can be created here that are aligned to a high degree because of the use of the digitalized printing process.
A further alternative that is known from the prior art is to directly create the required structure on the surface with the aid of a laser process; the deep structure to be produced is directly created with the aid of a laser. The laser is also precisely controlled with the aid of a digital printing technique with this method, so the press platens or endless belts are a 100% match with the decor paper after they are completed. The level required to create a match of the necessary press-platen structures with the decor paper has only been achieved with the processes for surface structuring that have recently been developed.
After the press platens or endless belts have been completed, they can be subjected to further process steps to obtain special effects, for instance to influence the degree of gloss. The degree of gloss provided can range from matte or silk matte to shiny; the wooden plates have a desired surface structure with a lustrous effect that comes very close to that of naturally grown wood because of this. Wood-pore structures are preferably produced with these methods, but imaginative surface designs can likewise be created or leather, tile or natural-stone surfaces can be imitated. Any conceivable structure can be created with this method.
The overlay paper could be doped with abrasion-resistant particles, for instance corundum, so that the end consumer has a high level of wear resistance depending on the intended use of the manufactured wooden plates, for instance in the wall area, but also in the floor area. But this wear resistance leads to negative effects on the press platens or endless belts that are used. There is a relative movement between the processed wooden material with the pressed surfaces and the press platen or endless belt after every pressing operation and an opening of the presses, so the surface structure is gradually worn down. It is necessary to completely rework the press platen or replace it with a new press platen in a case of that type. The press platens have to already be reconditioned after a relatively short period of use for coatings, for instance of HDF plates (high density fiberboard) and laminated floor coverings. The reason for that is the high proportion of corundum that is used in the melamine resin films to increase the wear resistance, which is why the chrome-plated surfaces wear down relatively quickly. The production of a press platen is very complicated and cost-intensive; the plan is to scrap them at the end of the service life. This is because of the fact, among other things, that the blanks of the press platens or endless belts are first provided with a direction-free, intermediate finish and have to additionally be polished ahead of the surface structuring before new structuring can take place, so the thickness of the press platens or endless belts will gradually no longer be at the required dimension after several instances of rework. The complex structure creation, for instance according to the classical etching method, application of the etching resist, etching with iron(III) chloride, as an example, creation of a degree of gloss via sandblasting, for instance, and subsequent surface sealing, for example chrome plating, or alternatively via the creation of the structure with the aid of laser engraving. Likewise, the press-platen surfaces usually have to be completely ground and reworked when there is mechanical damage. This leads to a situation in which the relatively expensive base press platens only remain conditionally operational, and they have to be scrapped after around four to six instances of reconditioning, because they become too unstable as a result of their thickness and cannot be fixed in place in a consistent way in the press system.